1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus, more particularly, to a magnetic recording medium having an a real recording density of 7.75 Gbit per square centimeter and a magnetic storage apparatus including the magnetic recording medium.
2. Discussion of the Background
Since 1998, the a real recording density of a hard disk drive is increased at an annual rate of 100 percent. However, the increase of a real recording density clearly causes the thermal decay that magnetically recorded data is erased under the influence of circumferential heat. In the conventional a real recording system, it is considered difficult to reach the a real recording density of over 7.75 Gbit per square centimeter.
Unlike the longitudinal recording system, the perpendicular recording system has the characteristic that the demagnetization field acting between adjacent bits is reduced as the linear recording density is increased, and the recorded magnetization maintains stable. Further, since a soft magnetic underlayer having a high permeability is arranged under a perpendicular recording layer, the strong recording magnetic field is obtained, thereby enabling the use of the perpendicular recording layer with high coercivity. Therefore, the perpendicular recording system is considered to be one of effective means which overcomes the thermal fluctuation limit of the longitudinal recording system.
In the perpendicular recording system, effectively, the high-density recording is realized by combining a single-pole-type head and a double-layer perpendicular recording medium comprising the soft magnetic underlayer and the perpendicular recording layer. However, since the double-layer perpendicular recording medium has the soft magnetic underlayer with a high saturation magnetic flux density (Bs), the leakage magnetic flux caused from a domain wall in the soft magnetic underlayer is observed as a spike noise. The motion of the domain wall in the soft magnetic underlayer cause the decay of the recorded magnetization and, further, the stray field in the apparatus concentrates to the recording head, thereby causing the decay of the recorded magnetization just under the recording head.
Japanese Laid-open Patent Application Publication No. 7-129946 (Patent Document 1) and Japanese Laid-open Patent Application Publication No. 11-191217 (Patent Document 2) disclose a method by which a hard magnetic pinning layer is provided between the soft magnetic underlayer and the substrate and the magnetization of the soft magnetic underlayer has one orientation. Further, Japanese Laid-open Patent Application Publication No. 6-103553 (Patent Document 3) discloses a method for suppressing the domain wall motion in the soft magnetic underlayer by the exchange coupling to the antiferromagnetism by aligning magnetic spins in a single direction.
According to the method for arranging the hard magnetic pinning layer, a magnetic domain is easily formed at the inner and outer edges of a disk, and the spike noise might be observed from the formed portion of the magnetic domain. On the other hand, according to the method for suppressing the domain wall motion in the soft magnetic underlayer by the antiferromagnetic layer, advantageously, the decay of recorded magnetization by the domain wall motion is suppressed and, however, the spike noise due to the domain wall is not suppressed.
Further, Japanese Laid-open Patent Application Publication No. 2001-155322 (Patent document 4) proposes a method for inverting the magnetization of the soft magnetic layer forming the soft magnetic underlayer containing two or more soft magnetic layers which are separated by the non-magnetic layer. However, when the substrate is disc-shaped, the layers easily have a multi-domain structure. The fluctuation of the recording and reproducing efficiency due to the domain wall is caused and the modulation in output signal is thus observed.